UK Patent Application GB 23744A describes an inductive position sensor in which a transmit aerial and a receive aerial are formed on a first member, and a resonant circuit having an associated resonant frequency is formed on a second member which is movable relative to the first member. An excitation signal having a frequency component at or near the resonant frequency of the resonant circuit is applied to the transmit aerial resulting in the generation of a magnetic field having a magnetic field component at or near the resonant frequency of the resonant circuit. The generated magnetic field induces a resonant signal in the resonant circuit, which in turn induces a sense signal in the receive aerial that varies with the relative position of the first and second members. The sense signal is processed to determine a value representative of the relative position of the first and second members.
In the position sensor described in GB 2374424A, the resonant signal induced in the resonant circuit is generated as a result of an electromotive force which is proportional to the rate of change of the magnetic field component at or near the resonant frequency. As the impedance of the resonant circuit is substantially entirely real at the resonant frequency, the resonant signal is approximately in phase with the electromotive force and accordingly is approximately 90° out of phase with the frequency component of the excitation signal near the resonant frequency. The sense signal induced in the receive aerial is generally in phase with the resonant signal, and therefore the sense signal is also approximately 90° out of phase with the component of the excitation signal near the resonant frequency of the resonant circuit.
The sense signal is synchronously detected using a signal which has the same frequency as, but is in phase quadrature with, the frequency component of the excitation signal near the resonant frequency of the resonant circuit. By using such phase sensitive detection, noise which is at the same frequency as, and is in phase with, the frequency component of the excitation signal near the resonant frequency of the resonant circuit is substantially removed along with noise at frequencies away from the resonant frequency.
A problem with such an inductive sensor is that noise can occur in the sense signal having components which have the same frequency as, but are in phase quadrature with, the component of the excitation signal near the resonant frequency of the resonant circuit. These noise components are not removed by phase sensitive detection and therefore affect the accuracy of the position measurement. Such noise components can be generated through signal coupling between components of the inductive position sensor, either directly or indirectly via a magnetically permeable body which is in close proximity with the inductive position sensor. This problem also arises in inductive position sensors in which a transmit aerial on a first member is directly coupled to a receive aerial, which includes a resonant circuit, on a second member.